1. Field of the Invention
The present invention relates to an optical cross-connect used in optical network and more particularly to a multicast-capable optical cross-connect with layered modularity.
2. Description of the Conventional Art
An optical network is composed of several nodes and optical fiber transmission links deployed between the nodes. Each node is equipped with an optical cross-connect and a plurality of monitoring and controlling devices. In optical networks, based on wavelength division multiplexing technology, a particular wavelength is allocated to respective links between the nodes in the network to establish an optical communication path.
For example, let""s suppose an optical network with 6 nodes as shown in FIG. 1 and 8 wavelengths, from xcex1 to xcex8, are available in the network. Then, the communication paths between the nodes are established as follows. xcex1 and xcex2 are allocated for connection between node 1 and node 2. xcex3 is allocated for connection between node 1 and node 3. xcex4 is allocated for connection between node 1 and node 4. xcex5 is allocated for connection between node 1 and node 5. xcex6 is allocated for connection between node 1 and node 6. xcex4 is allocated for connection between node 2 and node 3. xcex3 is allocated for connection between node 2 and node 4. xcex7 is allocated for connection between node 2 and node 5. xcex8 is allocated for connection between node 2 and node 6. The same wavelength is used for both sending and receiving signals in a node of the network. To promote efficiency, xcex3 used for connection between node 1 and node 3 is to be used again for connection between node 2 and node 4 and xcex4 used for connection between node 1 and node 4 is to be used again for connection between node 2 and node 3.
In optical network, optical cross-connect is installed at each node to control connectivity of optical paths between the nodes by means of wavelength routing as stated above. It drops signals whose destination node is the node where it is installed, provides optical paths to signals whose destination nodes are other nodes in the network, and adds signals generated by the node where it is installed.
For example, an optical cross-connect installed at node 1 provides optical paths for signals of xcex4 and xcex3 transmitted from node 2 to node 3 and node 4, respectively, and signals of xcex4, xcex3, xcex7, xcex8, from node 3, 4, 5, 6 to node 2. In addition, it drops signals of xcex1 and xcex2 transmitted from node 2 and signals of xcex3, xcex4, xcex5, and xcex6 transmitted from node 3, 4, 5, 6, respectively. It adds and provides optical paths for signals of xcex1 and xcex2 to node 2 and signal of xcex3, xcex4, xcex5, and xcex6 to node 3, 4, 5, and 6, respectively.
In FIG. 2, a conventional optical cross-connect that performs wavelength routing function in the network as stated above in accordance with previous invention is illustrated. This cross-connect has Mxe2x88x921 input fiber ports and Mxe2x88x921 output fiber ports. Each of them conveys N wavelength multiplexed signals of xcex1xcx9cxcexN. Mxe2x88x921 input fiber ports receive multiplexed signals and supply them to demultiplexers 111,112,113.
Demultiplexed signals are supplied to one of N space switches 121,122, 123, 124 as their wavelength and then N space switches establish optical paths for each signals. Added signals (Sadd) are supplied to N space switches and N space switches establish optical paths for them.
Output signals of space switches are supplied to Mxe2x88x921 multiplexers and signals to be dropped (Sdrop) are dropped.
Since conventional cross-connect employs a few elements such as demultiplexers, Mxc3x97M space switches, and multiplexers for corresponding functions like demultiplexing, routing, and multiplexing, its internal structure is fairly simple and clear. However, it has several drawbacks.
First, as the scale and capacity of the network increase, the number of fiber connected to the node in the network also increases. The size of multi-port space switches 121 to 124 in previously developed cross-connect depends upon the number of fiber ports connected to the node. Thus, as the scale and capacity of the network increase, switching time for establishing optical paths and insertion loss of the space switch increase. As a result, the previously developed cross-connect restricts scalability of the network.
Second, previously developed cross-connect causes difficulties in network evolution, node upgrade and maintenance because of its low modularity. As the network expands and thus the number of optical fiber ports in the cross-connect to be increase, whole node structure should be changed to accommodate such changes. In addition, if some parts of an Mxc3x97M space switch are not operating properly, the whole space switch module should be replaced. Thus, in order to fix some troubled parts of the cross-connect, whole network services should be down, including services that have nothing to do with the troubled parts of system.
Third, so as to guarantee the quality of signals above certain level, quality difference of signals like power inequality should be compensated. For doing this, devices like variable attenuator will be added to the input or output terminal of the switches. As a result, implementation of optical cross-connect becomes more expensive and its internal structure and control get complicated.
Fourth, in previously developed cross-connects, each input signals can be delivered to only one output fiber port and the number signals added or dropped is restricted to N at maximum. Therefore, it is not suitable for the node that requires flexible connectivity like multicasting.
By reasons stated above, previously developed cross-connects limits network scalability, causes difficulties in preparation for network evolution, node upgrade, and maintenance, and restricts its application to various network structures.
U.S. Pat. No. 5,627,925, May, 6, 1997, Non-blocking optical cross-connect structure for telecommunications network.
IEEE Journal of Lightwave Technology, Vol. 14, No. 6, June 1996, pp. 1410xcx9c1422, Satoru Okamoto, Atsushi Watanabe, and Ken-Ichi Sato, xe2x80x9cOptical Path Cross-connect Node Architectures for Photonic Transport Networkxe2x80x9d.
IEEE Journal of Lightwave Technology, Vol. 14, No. 10, October 1996, pp. 2184xcx9c2196, Eugenio lannone and Roberto Sabella, xe2x80x9cOptical Path Technologies: A Comparison Among Different Cross-Connect Architecturesxe2x80x9d.
The present invention provides an optical cross-connect that offers convenience for maintenance, network evolution, and node upgrade thanks to its layered modularity.
It provides capability of multicasting input signals to output fiber ports, adaptability to diverse networks that have different topologies. By employing simple elementary switches, control of optical cross-connect is simplified and switching time is minimized.
An optical cross-connect system for establishing connections between M+m input fiber ports 2M and output fiber ports comprises M+m 1xc3x97M optical power splitters, a drop link module, and M transmission link modules. The optical power splitters are connected to the input fiber ports and distribute input signals. The drop link module selects signals to be dropped at the node where the optical cross-connect is installed. The M transmission link modules select signals to be transmitted to a particular output fiber port among input signals.